Various heat-developable photographic materials are known. For example, such materials and photographic processes for processing them are described in Bases of Photographic Engineering, Edition of Nonsilver Photography (published by Corona Publishing Co., 1982), pages 242 to 255 and U.S. Pat. No. 4,500,626.
In addition, a method of forming a color image by a coupling reaction of an oxidation product of a developing agent and a coupler is described in, for example, U.S. Pat. Nos. 3,761,270 and 4,021,240. A method of forming a positive color image by a light-sensitive silver dye bleaching process is described in U.S. Pat. No. 4,235,957.
Recently, a dye transfer method of imagewise releasing or forming a diffusible dye by heat development followed by transferring the diffusible dye to a dye-fixing element has been proposed. In accordance with such method, both a negative color image and a positive color image can be obtained by varying the kind of the dye-donating compound used or the kind of the silver halide used. The details of such a method are given in U.S. Pat. Nos. 4,500,626, 4,483,914, 4,503,137, 4,559,290, JP-A-8-149046, JP-A-60-133449, JP-A-59-218443, JP-A-61-238056, EP-A-220746, Japanese Disclosure Bulletin 87-6199 and EP-A-210660. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
Various methods have also been proposed for forming positive color images by heat development. For instance, U.S. Pat. No. 4,559,290 proposed a method in which an oxidized compound having no color image releasing capacity as derived from a DRR compound is, in the presence of a reducing agent or a precursor thereof, reduced with the reducing agent which remains without being oxidized after oxidation of the agent in accordance with the exposed amount of a silver halide by heat development, to release a diffusible dye. EP-A-220746 and Japanese Disclosure Bulletin 87-6199 (Vol. 12, No. 22) describe a heat-developable color photographic material comprising a compound which releases a diffusible dye by the same mechanism as mentioned above, that is, the compound releases a diffusible dye by reductive cleavage of the N--X bond of the compound (where X is an oxygen atom, a nitrogen atom or a sulfur atom).
Since such heat-developable photographic materials may be processed in a short period of time and may be processed with a simple developing apparatus, they are now in common use. With their increased popularity, they are used in many different kinds of places, and thus they have been found to involve the problem that their sensitivity often fluctuates due to fluctuations in the temperature of the place where they are used.
In order to further shorten the processing time of heat developable photographic materials, an exposure method using laser rays is in utilized. However, since exposure with laser rays is effected in a short period of time with higher light intensity than ordinary optical exposure, it involves the problem of high intensity reciprocity failure in that the sensitivity fluctuates due to variations in the light intensity imparted to photographic materials upon exposure thereof.
JP-A-51-139323 and JP-A-59-171947 and GB-A-2109576 mention that the reciprocity law failure may be improved by incorporation of a compound of a metal of Group VIII into photographic materials. JP-B-49-33781 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-50-23618, JP-A-52-18310, JP-A-58-15952, JP-A-59-214028, JP-A-61-67845, German Patents 2226877 and 2708466 and U.S. Pat. No. 3,703,584 mention that an improvement of the reciprocity law failure may be effected by incorporation of a rhodium compound or an iridium compound into photographic materials. However, incorporation of a rhodium compound causes extreme desensitization of a photographic material containing it, which is unfavorable to practical use. On the other hand, incorporation of an iridium compound often involves latent image sensitization which increases the density of developed images with the passage of time from exposure of the photographic materials to processing, which is also unfavorable to practical use.